Solar overall water-splitting by a spin-hybrid all-organic semiconductor

Direct solar-to-hydrogen conversion from pure water using all-organic heterogeneous catalysts remains elusive. The challenges are twofold: (i) full-band low-frequent photons in the solar spectrum cannot be harnessed into a unified S1 excited state for water-splitting based on the common Kasha-allowed S0 → S1 excitation; (ii) the H+ → H2 evolution suffers the high overpotential on pristine organic surfaces. Here, we report an organic molecular crystal nanobelt through the self-assembly of spin-one open-shell perylene diimide diradical anions (:PDI2-) and their tautomeric spin-zero closed-shell quinoid isomers (PDI2-). The self-assembled :PDI2-/PDI2- crystal nanobelt alters the spin-dependent excitation evolution, leading to spin-allowed S0S1 → 1(TT) → T1 + T1 singlet fission under visible-light (420 nm~700 nm) and a spin-forbidden S0 → T1 transition under near-infrared (700 nm~1100 nm) within spin-hybrid chromophores. With a triplet-triplet annihilation upconversion, a newly formed S1 excited state on the diradical-quinoid hybrid induces the H+ reduction through a favorable hydrophilic diradical-mediated electron transfer, which enables simultaneous H2 and O2 production from pure water with an average apparent quantum yield over 1.5% under the visible to near-infrared solar spectrum.

Direct solar-to-hydrogen conversion from pure water using all-organic heterogeneous catalysts remains elusive.The challenges are twofold: (i) full-band low-frequent photons in the solar spectrum cannot be harnessed into a unified S 1 excited state for water-splitting based on the common Kasha-allowed S 0 → S 1 excitation; (ii) the H + → H 2 evolution suffers the high overpotential on pristine organic surfaces.Here, we report an organic molecular crystal nanobelt through the self-assembly of spin-one open-shell perylene diimide diradical anions (:PDI 2-) and their tautomeric spin-zero closed-shell quinoid isomers (PDI 2-).The self-assembled :PDI 2-/PDI 2-crystal nanobelt alters the spindependent excitation evolution, leading to spin-allowed S 0 S 1 → 1 (TT) → T 1 + T 1 singlet fission under visible-light (420 nm~700 nm) and a spin-forbidden S 0 → T 1 transition under near-infrared (700 nm~1100 nm) within spin-hybrid chromophores.With a triplet-triplet annihilation upconversion, a newly formed S 1 excited state on the diradical-quinoid hybrid induces the H + reduction through a favorable hydrophilic diradical-mediated electron transfer, which enables simultaneous H 2 and O 2 production from pure water with an average apparent quantum yield over 1.5% under the visible to near-infrared solar spectrum.
Extracting hydrogen (H 2 ) from pure water powered by clean and inexhaustible sunlight is a long-dreamed solar energy conversion strategy in a way that compensates for the intermittency of solar irradiation 1,2 .Such technology requires a photocatalyst of superior visible-to-NIR light response that is maximally close to the solar spectrum, long-enough exciton lifetimes for ns-scale water splitting, and proper band structure for both hydrogen and oxygen evolution reactions (i.e., HER and OER) 3,4 .Although much desired, currentdeveloped semiconductor catalysts, including transition-metal oxides 5 , -oxynitride 6 , metal-organic hybrid framework-based materials 7 , and metal-free inorganic semiconductors (e. g., C 3 N 4 and BN) 8,9 , cannot meet all the requirements for commercial application with >10% solar-to-hydrogen (STH) conversion efficiency 10,11 .Besides the commonly low apparent quantum yields (AQYs) at the wavelength ≥420 nm visible-to-NIR region of incident sunlight 4 , current stereotypical photocatalyst designs require the loading of expensive or poisonous noble-metal/metal-oxide as co-catalysts to help release H 2 and O 2 1-4   .In this sense, metal-free all-organic catalysts capable of solar overall water-splitting hold research perspectives 11 .
Compared with metal-based inorganic or organic-inorganic complex photocatalysts, covalent C-H-X organic materials available for various structural tailor and functional designs show superior advantages in light-harvesting initialed by π → π or n → π transitions 11 .Some even reach the world record for HER half-reaction efficiency over inorganic catalysts 12 .However, due to the fast radiative recombination of direct S 0 → S 1 excitations 13,14 and the thermodynamical energy requirement of overall water-splitting 4 , direct STH conversion over common organic materials is impossible with low-frequency photons around 420-1000 nm, ~50% energy of the incident solar spectrum 15 .As a solution, the upconversion strategy based on the triplet-fusion can double the incident photon energy, driving the water-splitting reaction that is thermodynamically impossible to direct low-frequency irradiation 16 .Compared to the Kasha-allowed singlet excitation (S 0 → S 1 ), the triplet-fusion pathway is mediated by a long-lived triplettriplet (TT) exciton pair 16 , which is inaccessible by a direct S 0 → T 1 transition due to the disallowed spin 13 .Consequently, dark-state TT pairs of an organic chromophore in a sensitizer-annihilator system can only be indirectly populated either by the S 1 → T n intersystem crossing (ISC, n ≥ 1) (Fig. 1a) or the S 0 S 1 → 1 (TT) → (TT) singlet fission (SF) process 14,17,18 , which generally requires strong UV/vis absorption to produce the initial singlet excited state on the sensitizer and a robust conjugation of the sensitizer-annihilator chromophore pair 19 .This is incompatible with utilizing full-band low-frequency photons in the solar spectrum.Moreover, as a competing pathway to the Dexter triplet-triplet energy transfer (TTET), high-energy triplet excitons arose from little S 1 /T 1 energy difference are easily quenched by triplet O 2 20 , the main product of the water-splitting reaction (H 2 O → H 2 + O 2 ), to produce singlet O 2 , rendering STH conversions unsustainable.In addition, due to the common high overpotential on simple hydrocarbon surfaces, the H + → H 2 evolution reaction (HER), a key halfreaction in water splitting, can hardly occur on organic catalysts unless loaded with noble-/transition-metal-based cocatalysts.
In contrast, a triplet-fusion pathway with TT pairs from intrinsic S 0 → T 1 transition can harness a wide range of incident sunlight, including near-infrared (NIR) photon energy, and be immune to O 2 quenching with low triplet energy levels.Until now, such a spinforbidden S 0 → T n transition, however, has only been achieved in a few examples by spin-orbit coupling with heavy atoms (e.g., Lanthanide, Iodine, Osmium, Platinum, and Gold) [21][22][23][24][25] .Although much desired, allorganic material-induced direct S 0 → T n transition without any heavy atoms has yet to be identified.We envision the direct S 0 → T 1 transition by a robust spin-orbit coupling between the preset open-shell triplet chromophore (i.e., diradical) and the singlet closed-shell isomer chromophore within a heavily conjugated crystal.Such a design not only makes the full use of the visible-to-NIR solar spectrum by the twoband excitation but also creates a proton-favorable electron transfer channel by diradical-dominated reactive interface on all-organic catalysts to break through the HER bottleneck (see Fig. 1b).

Synthesis and catalyst structure
We opt N, N'-di(propanoic acid)-perylene-3, 4, 9, 10-tetracarboxylic diimide (denoted PDI throughout the article) (Fig. 2g) as the model perylene diimide parent to prepare the target spin-hybrid all-organic semiconductor (for details of the preparation methods, see Method).The pristine PDI in aqueous solution exhibits broad visible-light absorption with double peaks at 460 nm and 596 nm (Fig. 2a).After the hydrazine reduction (N 2 H 4 + 2PDI → N 2 + 2 :PDI 2− /PDI 2− + 4H + ), :PDI 2− diradical anions and corresponding PDI 2− quinoid tautomer are generated with an emerging sharp absorption profile peaking at 530 nm and 575 nm.Distinct color difference between PDI and :PDI 2− /PDI 2− aqueous solutions can be distinguished (Supplementary Fig. 2).Electron spin-resonance (ESR) spectra show an intense radical spin feature at g = 2.0032 (Fig. 2b), indicating a triplet diradical feature of :PDI 2− .A number of qualitative spectral characteristics (Supplementary Figs.2-7) verify the :PDI 2− /PDI 2− formation and the structure with reduced cis-/trans-carbonyls (Fig. 2g) and the simplest protonation form of (H + ) 2 -(:PDI 2− /PDI 2− ).Note that after the two-electron reduction, singlet photoluminescence (PL) emissions at 540 nm and 585 nm corresponding to the radiative relaxation of S 1 → S 0 can be observed over the :PDI 2− /PDI 2− solid sample (Fig. 2d), suggesting the coexistence of closed-shell PDI 2− quinoid isomers (S 0 ) and open-shell :PDI 2− diradicals (T 1 ) through tautomerism.Compared to PDI, :PDI 2− /PDI 2− exhibits a much smaller Stokes shift (~15 nm) (Supplementary Fig. 8), suggesting a smaller reorganization energy than that of PDI.This is further demonstrated by smaller changes and shifts in infrared vibrational intensity on :PDI 2− /PDI 2− than PDI under the same excitation condition (Supplementary Fig. 9).Triplet states predominantly decay through non-radiative processes or much slower radiative processes like phosphorescence, resulting in the non-fluorescent :PDI 2− diradicals as a 'dark state'.Upon reduction by N 2 H 4 •H 2 O, the fluorescence peak of PDI rapidly decreases to zero within 10 s, resulting in the formation of nonfluorescent :PDI 2− triplet diradical anions (Fig. 2e).However, within the next 30 min, fluorescence peaks at 646 nm and 690 nm gradually emerge, indicating the transition from non-fluorescent :PDI 2− to fluorescent PDI 2− via tautomerism transformation.Such a tautomeric conversion of ːPDI 2− → PDI 2− after the 2e − reduction of PDI was further in-situ tracked by the increasing PL emission in 30 min (Fig. 2e).After completely removing residual hydrazine hydrate from the solvent, singlet emissions of the quinoid recovered to 540 nm and 585 nm (Fig. 2f), identical to the solid sample (Fig. 2d).This means it is the coexisted hydrazine hydrate that makes the peaks of PDI 2− significant redshift from 540 nm and 585 nm to the 641 nm and 688 nm, as shown in Fig. 2e.To further validate this, we performed additional control experiments by adding varying amounts (0~0.35μL) of hydrazine hydrate to the DMF solution of the DMF solution of isolated :PDI 2− / PDI 2− sample.As expected, we observed a gradual redshift in the PL emission of :PDI 2− /PDI 2− (Supplementary Fig. 10), validating that the presence of hydrazine hydrate in the solution indeed causes a significant redshift in the fluorescence of :PDI 2− /PDI 2− .The diradical/quinoid ratio is determined by the integral PL emission intensity of :PDI 2− / PDI 2− compared with PDI (:PDI 2− is non-fluorescent), suggesting a :PDI 2− /PDI 2− ratio of 74% to 26% in the precursor samples at room temperature (15 °C) (Fig. 2f).The solid :PDI 2− /PDI 2− precursor for subsequent self-assembly can be obtained by evaporating the solvent and stabilized under anaerobic conditions.
Due to the solid π-π interaction of perylene rings between individual PDI molecules, the simple phase-transition method has enabled the controllable self-assembly of single-phase PDI-based organic semiconductors with regular morphologies and delicate crystal structures (Supplementary Fig. 11) 26,27 .Applying the same selfassembly strategy, the self-assembly of :PDI 2− /PDI 2− was performed through the phase conversion in an optimized mixture of ethanol (poor solvent) : water (good solvent) = 1 : 1 under anaerobic conditions at 60 °C.Within 120 h, a gradual morphology transition of nanor-od→nanowire→nanobelt can be observed with the disappearance of irregular structural fragments (Fig. 3a).Extending the self-assembly time to 120 h renders the formation of single-crystalline nanobelts with a length ≥1 mm (Fig. 2c), the width of ca.~500 nm (Fig. 3a), and the thickness of ca.~250 nm (Fig. 3b).The morphology of self-assembled :PDI 2− /PDI 2− nanobelt no longer changes after 120 h.Separated by centrifugation, obtained all-organic semiconductors remain stable in solvents, including methanol, ethanol, and water, or as a powder for more than half a year without further morphological and structural transitions (Supplementary Fig. 12).
Typically, the self-assembly of PDI derivatives occurs via the strong π-interaction between perylene rings, forming a vertical face-toface planar stacking H-type configuration.In contrast, some polar group-terminated PDI molecules can follow a J-type arrangement in a rotational head-to-tail stacking model.N, N'-di(propanoic acid)-perylene-3, 4, 9, 10-tetracarboxylic diimide is well-reported that follows the H-type configuration 28 .However, albeit sharing an identical molecular framework with PDI, :PDI 2− /PDI 2− aggregates show a distinct J-type pattern (Supplementary Figs. 13, 14 and Supplementary Table 1) with redshifted visible-light absorption after the self-assembly due to πstacking splitting (see below).Combined with the single-crystal selected area electron diffraction (SAED) pattern (Fig. 3c), a simulated crystal model of :PDI 2− /PDI 2− organic semiconductor can be visualized (Fig. 3d).Notably, after the self-assembly, the fluorescence emissions of the PDI 2− quinoid isomer can still be observed (Supplementary Fig. 15), indicating that the tautomeric equilibrium between :PDI 2− and PDI 2− maintains in self-assembled crystals.
Moreover, after the self-assembly, the lattice parameter of ːPDI 2− / PDI 2− crystal evolves as the {100} peak shifts towards larger angles in 120 h (Fig. 3e).Such a shift epitomizes simultaneous features combining the disappearance of irregular fragments and the growth of selfassembled crystals, corresponding to a shortened apparent lattice length along the a-axis of the self-assembled crystal (Fig. 3f).Meanwhile, the ESR feature of ːPDI 2− also shifts with the self-assembly time (Fig. 3e).After 96 h of self-assembly, a new splitting feature at g = 2.0044 and g = 2.0037 corresponding to the non-zero field transition of the spin-triplet state (STS) emerge, which implies a solid spinorbit coupling within triplet ːPDI 2− diradicals.Such a triplet spin-orbit coupling can occur on neighboring diradicals or between quinoids and diradicals, forming the triplet-singlet diradical-quinoid (D-Q) hybrid state.Since a shortened a-length from strain narrows the intermolecular distance between building blocks, the interaction between unpaired electrons can be enhanced, leading to an intermolecular triplet spin-orbit coupling within the patterned self-assembled ːPDI 2− / PDI 2− crystal.Moreover, a broad NIR absorption band peaking at ~700 nm emerged after the spin coupling, which broadened the visible-light absorption range of self-assembled ːPDI 2− /PDI 2− nanobelt close to the profile of the solar spectrum (Fig. 4c).Such an extra NIR absorbance corresponds to an unusual direct S 0 → T n transition that is typically spin-forbidden in organic molecular chromophore systems.Due to the forbidden nature of the immediate optical transition between the spin-zero ground (S 0 ) state and the spin-one triplet (T 1 ) levels, molecular triplet exciton is a dark state with no direct triplet absorption 29,30 .However, pioneering works that couple organic excimers with unpaired spins of heavy atoms such as Lanthanide enable a bright triplet NIR absorption through the triplet energy transfer [21][22][23][24][25]29,30 . In ou system, the NIR absorption occurs in the interval of enhanced triplet spin-orbit coupling (72~96 h) (Fig. 5a), consistent with the feature of the direct triplet excitation of S 0 → T n transition on the D-Q hybrid.Such a direct S 0 → T 1 transition not only activates 'dark' triplet :PDI 2− diradicals to the NIR absorbance, but also provides the opportunity to double the energy of these formed triplet excitons through a TTA upconversion for water-splitting reaction.

Photocatalytic overall water-splitting
Figure 4a shows the quantified evolution rates of H 2 and O 2 from 50 mL water containing 0.05 g ːPDI 2− /PDI 2− catalysts with different selfassembly times under the constant white light illumination (Xe lamp, central spot intensity 1000 mW cm -2 ), in which H 2 and O 2 productions can be observed without introducing any metal-based cocatalysts.The discrepancy in the H 2 /O 2 deviating from the ideal 2 :1 stoichiometric ratio is from the inevitable H 2 O 2 production, which accounts for approximately 60 µmol g −1 h −1 (Supplementary Fig. 16 quantified by the I -titration method).The performance of ːPDI 2− /PDI 2− catalysts has been improved in order of magnitudes after the self-assembly.Notably, a leap of the performance (~2.5 times) can be observed on ːPDI 2− /PDI 2− catalysts between 72 h and 96 h of self-assembly (Fig. 4a), consistent with the interval of triplet spin-orbit coupling (Fig. 3e).Further extending the self-assembly time to 120 h shows the best hydrogen production rate (HPR).As controls, water-splitting experiments on PDI before and after self-assembly were performed with no H 2 and O 2 formation detected (Supplementary Fig. 17).Isotopically labeled  experiments were further conducted to confirm that observed H 2 and O 2 productions were sorely from the photocatalytic water splitting rather than other effects (Fig. 4d, Supplementary Figs.18-22).A turnover number (TON) > 19 was achieved within 150 h by performing the long-term reaction over the self-assembled ːPDI 2− /PDI 2− nanobelt catalyst, solidly verifying the occurrence of overall water splitting (Fig. 4e).Due to the intense visible-to-NIR absorption in the range of 450 nm~850 nm, self-assembled ːPDI 2− /PDI 2− nanobelt catalyst exhibits superior efficiencies under the AM 1.5 G simulated sunlight (1 sun, 100 mW cm -2 ) (Fig. 4b) and apparent quantum yields (AQY) in the visible-to-NIR region (Fig. 4c).The ːPDI 2− /PDI 2− nanobelt catalyst exhibits a record AQY of 1.96% at 550 nm and the average AQY exceeds 1.5% in the range of 550-850 nm, the most intense radiation band of the solar spectrum (Supplementary Tables 2-4, performed once).More importantly, with the λ = 850 nm excitation, an AQY of 0.95% was obtained.Such a single-band NIR-driven water-splitting is impossible with direct excitations by low-energy photons (<1.5 eV) due to the overpotential and thermodynamical requirement [1][2][3][4] .The STH efficiency of the ːPDI 2− /PDI 2− nanobelt catalyst was determined to be ~0.0806%,which is the first example of solar water splitting over allorganic materials.Our self-assembled ːPDI 2− /PDI 2− catalyst only stably works under continuous vacuum pumping to avoid the excessive accumulation of O 2 (Fig. 4e), which can slowly corrode the material.

Mechanism analysis
We attribute the solar overall water-splitting efficiency over the selfassembled ːPDI 2− /PDI 2− catalyst to the following two aspects : i) accommodating the two-band excitation, i.e. spin-allowed SF and spinforbidden S 0 → T 1 transition, to coherently utilize the full visible-to-NIR solar spectrum to produce triplet excitons for TTA-based upconversion; ii) decreased HER overpotential by hydrophilic diradical ːPDI 2− moiety with highly delocalized unpaired electrons.We first investigate the excitation feature through the steady-state PL emissions of ːPDI 2− / PDI 2− solid samples with different self-assembly times.Under 450 nm excitation (Fig. 5b), ːPDI 2− /PDI 2− samples only exhibit singlet emissions of the quinoid moiety at 540 nm and 585 nm with self-assembly times of 24 h, 48 h and 72 h.However, with the increasing self-assembly times, ːPDI 2− /PDI 2− samples from 84 h to 120 h additionally show a broad and intense emission band transitioning from 654 nm to 729 nm on the original quinoid singlet emission feature (Fig. 5b), indicating a new S 1 state in sharp contrast to that of the quinoid moiety.Such a broad PL emission appears on ːPDI 2− /PDI 2− samples after 84 h of self- assembly, coinciding with the spin coupling threshold (Fig. 3e).Due to the unchanged PL feature of the quinoid moiety and the nonluminescent property of ːPDI 2− diradicals, we ascribe the newly formed PL emission to the TTA upconversion on the D-Q hybrid (Fig. 5b).Under 450 nm excitation, such a TTA upconversion are only available on well-assembled crystals, which we believe arise from the spin-allowed S 0 S 1 → 1 (TT) SF on the diradical chromophore and subsequent 1 (TT) → T 1 + T 1 dissociation process to produce the final T 1 state (diffusive in the D-Q hybrid).On the 120 h self-assembled ːPDI 2− / PDI 2− nanobelt, the quantum efficiency for the TTA upconversion emission (λ em = 719 nm) was measured as 0.688%.Typically, twophoton or multi-photon upconversion processes exhibit nonlinear behaviors 31 .However, in our system, the triplet fusion upconversion (TFU) occurs immediately following SF, which differs from the conventional upconversion mechanisms.We further performed PL emission spectra measurements with varying light intensities (10 µJ cm −2 ~30 µJ cm −2 ) (Supplementary Fig. 23a).The observed linearity does not conform to the typical two-photon upconversion mode, showing a linear relationship (Supplementary Fig. 23b), which is consistent with our SF-TFU transition route.The SF and TFU processes are inversely related and interconnected, leading to a less pronounced nonlinear behavior.Similar phenomenon has been observed on other organic photovoltaic cells that employs a pentacene-based SF process with a sublinear dependence on excitation intensity, which is distinct from the typical nonlinear response in conventional multi-photon upconversion systems 32 .Notably, the lifetime of the upconversion emission remains unchanged with different incident light intensities (Supplementary Fig. 23c), suggesting that no saturation or intensityinduced quenching (such as exciton-exciton annihilation or energy pooling) occurs during this process.When same samples were measured under 808 nm excitation, an identical transitioning PL emission from 645 nm to 705 nm after 84 h of self-assembly was observed with singlet emissions of the quinoid moiety at 540 nm and 585 nm disappeared as expected (Fig. 5d).Since the S 1 state of both ːPDI 2− and PDI 2− chromophores are above the 808 nm excitation energy (Fig. 5a), triplet excitons are no longer produced via SF, but from a direct S 0 → T 1 transition on the D-Q hybrid.This provides solid evidence for the selfassembly-induced triplet-fusion upconversion.
Furthermore, we conducted femtosecond transient absorption spectroscopy (fs-TAS) measurements on the :PDI 2− /PDI 2− samples before and after 120 h self-assembly.The 120 h self-assembled :PDI 2− / PDI 2− sample showed a rapid singlet fission (SF) process (occurring in 300 fs) and the emergence of a triplet excited state absorption (ESA) feature (around 770 nm) after SF upon λ ex = 540 nm excitation (Supplementary Fig. 24d-g), with the SF yield calculated to be approximately 42%.This observation contrasts with the behavior of the :PDI 2− / PDI 2− molecular precursor (Supplementary Fig. 24a-c).An identical triplet ESA at ~770 nm, resulting from the direct S 0 -T 1 transition, was also observed under λ ex = 700 nm excitation (Supplementary Fig. 24h-j).Detailed data analysis and signal assignment was described in Supplementary Information.Our fs-TAS observations are consistent with our photoluminescence (PL) emission experiments and strongly support our proposed mechanism.
The transitioning redshift of the TTA-upconversion emission is due to the enhancing spin coupling between diradicals and quinoids with the increasing self-assembly times, similar to that in the rubrenelanthanide system 21 .Furthermore, the upconversion emission of the 120 h self-assembled sample at 729 nm has a much longer lifetime (τ = 4.33 ns) than that at 540 nm (τ = 0.67 ns) and 585 nm (τ = 0.79 ns) (Fig. 5c).A longer emission decay time directly corresponds to a slower back-electron-transfer rate 33 , which facilitates the ns-scale interfacial water-splitting reaction.This new S 1 state on the D-Q hybrid with a maximum 729 nm emission indicates that the energy of the parent T 1 state is lower than 0.85 eV, which is below the 3  .More importantly, the observed TTA upconversion emission could be severely extinguished by water (Fig. 5e), while singlet emissions at 540 nm and 585 nm of quinoid moiety could not.This indicates that the new S 1 excited state on the D-Q hybrid from TTA-upconversion is water-reactive, consistent with the unusual NIR-driven water-splitting efficiency on ːPDI 2− /PDI 2− nanobelt.We further conducted temperature-dependent PL measurements on the :PDI 2− /PDI 2− (120 h) sample over 100 K to 300 K (Supplementary Fig. 25a).Through the Arrhenius kinetic analysis, the binding energy (E b ) of *S 1 singlet excitons after the TTA upconversion is determined as 21 ± 2 meV (Supplementary Fig. 25b), which is significantly lower than the binding energies typically observed in other organic semiconductors.This value is less than the E b of the S 1 state associated with the 540/ 580 nm emission and the K B T value (~26 meV) at room temperature, suggesting that TTA-resulted *S 1 singlet excitons are highly active and readily dissociate into electrons and holes to participate in reactions, which is consistent with the fluorescence quenching in water (Fig. 5e).The self-assembly of such spin-coupled ːPDI 2− /PDI 2− chromophore renders intense adsorption features in the NIR region (700 nm~1100 nm) (Fig. 5a), matching well with the direct S 0 → T n transition feature with 0-0, 0-1, and 0-2 vibrational states [21][22][23][24][25] .However, with the 980 nm excitation (~lowest 0-0 vibrational level), the upconversion emission on samples did not appear (Fig. 5f), implying that the TTA-upconversion requires photon energies sufficient for the S 0 → T 1 transition.
Besides the two-band excitation to make the full use of the visibleto-NIR solar spectrum, breaking through the HER bottleneck with even a much lower S 1 state level on the D-Q hybrid than that either on ːPDI 2− or PDI 2− monomer chromophore is another important issue.A radicalmediated electron transfer (ET) pathway is reported to hold much lower reaction overpotentials, allowing reactions that are impossible under the non-radical pathway to occur 35,36 .In our system, the interfacial ET of HER occurs on the ordered ːPDI 2− /PDI 2− surface with plenty of diradical unpaired electrons, which makes the H + → H 2 reduction easier along the favorable diradical-mediated pathway.In addition, the enhanced hydrophilicity of ːPDI 2− /PDI 2− nanobelt may also play an important role.Unlike the generally inert covalent H-coordination of organic surface, ːPDI 2− /PDI 2− nanobelt has an ionic diradical dipole with enhanced affinity with protons to reach the charge balance, forming a hydrophilic surface with better wettability (Supplementary Fig. 26), which is essential for the adsorption and dissociation of H 2 O molecules at the water/catalyst interface.Moreover, the small E b of TFU-resulted *S 1 excitons (21 ± 2 meV) makes the reduced :PDI 2− diradicals exhibit properties more akin to inorganic materials, contributing to the breaking through of HER bottleneck without noble-/transition-metal components.
Our work is the proof-of-concept example of a metal-free organic catalyst capable of vis-to-NIR-driven overall water-splitting.Furthermore, such a self-assembly strategy of open-shell/closed-shell resonant structure applies to multiple derivatives of the PDI family (five derivatives verified) (Supplementary Fig. 27).This provides a path to design a series of brand-new photo-response organic semiconductors for photocatalysis, solar cells, sensors and light emitting materials.

Preparation of PDI
In a typical preparation procedure, 1.376 g (3.507 mmol) of perylene-3,4,9,10-tetracarboxylic dianhydride, 2.5 g (28.06 mmol) of β-Alanine, 18 g of imidazole were placed in a mortar and evenly grounded.Immediately transferred to a porcelain boat, heated in a tube furnace at 100 °C for four hours under the protection of Nitrogen.The resulting solid product was then uniformly dispersed in 100 mL of ethanol with the addition of 300 mL of 2 M HCl.After magnetically stirring overnight, the resulting mixture was centrifuged and washed with deionized water until the solution was neutral (pH ~7).Obtained solid was collected and dried at 60 °C in the vacuum for further use.
Preparation of :PDI 2− /PDI 2− precursor In a typical preparation procedure, 1.0 g of freshly prepared PDI was first dissolved uniformly in 100 mL hydrazine hydrate (>85%).The reduction reaction should last for more than 4 h.Then evaporating the solvent with the protection of Nitrogen flow.Obtained solid was collected and washed with ethanol, and dried on a 60 °C hot stage until constant weight (in a Nitrogen glove box).The obtained product is a mixture of open-shell diradical and the tautomeric closed-shell quinoid isomer with a ratio of 74% to 26%, denoted :PDI 2− /PDI 2− , which can be stably stored under the anaerobic condition.
Self-assembly of ːPDI 2− /PDI 2− nanobelt In a typical preparation procedure, 10 mg ːPDI 2− /PDI 2− precursor was dissolved uniformly in 5 mL of deionized water (good solvent), then 5 mL ethanol (poor solvent) was added.The mixed solution was transferred into a 10 mL vial.The vials were sealed and placed on a 60 °C hot stage for 0-120 h.The samples with different self-assembly times were centrifuged and washed with water and dried until constant weight.The above operations were carried out in the Nitrogen glove box.

Water-splitting performance test
The photocatalytic water-splitting over different catalysts was carried out in an outer irradiation-type photoreactor (quartz glass) connected to a closed gas-circulation system.In a typical procedure, 0.05 g of the photocatalysts were dispersed by a magnetic stirrer in 50 mL of deionized water.The catalyst was transferred from the glove box by injection sampling to avoid oxidation of the catalyst.The suspension is thoroughly degassed to remove air and irradiated using a 300 W Xelamp (1000 mW cm −2 , CEL-HXF300-T3, Beijing China Education Aulight Technology Co. Ltd, China).The photocatalytic H 2 and O 2 evolution rate was analyzed using an online GC-7920 gas chromatograph (GC, TCD detector, 5 A˚molecular sieve columns and Ar 2 carrier).Magnetic stirring (400 rpm) was used during the water splitting experiment to ensure homogeneity of the suspension and to eliminate sedimentation.With simulated AM1.5 G sunlight irradiation, a solar simulator (100 mW cm −2 , Xenon Light Source CEl-S500-T5, Beijing China Education Au-light Technology Co. Ltd, China) was employed as the light source.

Isotopically labeled GC-MS measurement
Preparation of isotopically labeled samples.In the Nitrogen glove box, 20 mg ːPDI 2− /PDI 2− nanobelts was dispersed uniformly in 10 mL of D 2 O (H 2 18   O or H 2 O) and transferred into a 50 mL volume vial.The gas (5 mL) was collected by the injection needle with a throttle valve into sealed gas bag, denoted as Light-off sample.The vials were sealed and irradiated using a 300 W Xe-lamp (1000 mW cm −2 , CEL-HXF300-T3, Beijing China Education Au-light Technology Co. Ltd, China) for 2 h.The reaction gas (5 mL) was collected by the injection needle with a throttle valve into sealed gas bag, denoted as Light-on sample.The Light-off samples and Light-on samples were collected from the same vial.
Using agilent CP-Molsieve 5A as a column for isotope-labeled samples analysis The 2 mL gas samples were collected and manually injected by gastight syringes (the FTFE luer lok (PN5190-1534), 0~2.5 mL) and then analyzed by gas chromatography-mass spectrometry (8890-5977B GC-MS instrument, Agilent Technologies, USA) equipped with an Agilent CP-Molsieve 5A column (25 m × 250 μm × 30 μm, Agilent Technologies, USA) in GC-MS.Helium was used as carrier gas.The column was maintained at 100 °C for 5 min, and the flow of the carrier was 1.5 mL min −1 .The temperatures of the injector and MS source were set to be 100 °C and 230 °C, respectively.The ionization potentials used for analysis were set to 20 eV and 70 eV, respectively.
In addition, we repeated the isotopically labeled GC-MS measurement using headspace vial (sample through drainage method) under above mentioned chromatographic conditions.The ionization potentials used for analysis were set to 18 eV and 70 eV, respectively.

Fe(III)-1, 10-Phenanthroline spectrometric titration measurements
The reductive electron concentration on ːPDI 2− /PDI 2− precursor was measured by a Fe(III)-1, 10-phenanthroline titration spectrometric method.1, 10-phenanthroline spectrometric measurement is a simple and widely used method for measuring Fe(II) ions.Here, we use the Fe(III) solution to titrate the unpaired electrons on ːPDI 2− /PDI 2− precursor that quantitatively lead to the production of Fe(II) ion, then used 1, 10-phenanthroline to measure the concentration of Fe(II) ions.Thus, we can quantitatively obtain the concentration of trapped electrons.The concentration of Fe(NO 3 ) 3 solutions employed in this measurement is 10 −3 M. 0.2% 1, 10-phenanthroline aqueous solution and pH = 4.6 HAc-NaAc buffer solution were previously prepared for use.The pH=4.6 HAc-NaAc buffer solution was prepared by dissolving 135 g sodium acetate and 120 mL acetic acid into 500 mL water solution.Before the titration, the Fe(NO 3 ) 3 solution is purged by nitrogen for 30 min to remove oxygen.The titration was conducted in the glove box.In a typical procedure, 2.5 mL ːPDI 2− /PDI 2− aqueous solution was taken in the glove box and mixed with 2.5 mL Fe(NO 3 ) 3 solution.5 min later, the resulting mixed solution was taken out to conduct the spectrometric measurement.The spectrometric measurement was conducted in the air.1.5 mL supernatant taken from the glove box was mixed with 1.5 mL pH = 4.6 HAc-NaAc buffer solution, and then added with 1 mL 0.2% 1, 10-phenanthroline water solution to obtain a red solution.Waiting 5 min to obtain a stable state, the resulting solution was transferred in a quartz cuvette and measured on a Hitachi U3900 spectroscopy.Obtained absorbance value was compared with the standard fitting line to obtain a certain Fe(II) concentration.The background data was collected by the same method except replacing 1, 10-phenanthroline solution with deionized water to eliminate the influence of PDI absorbance.

Characterization devices
The crystalline structure of the resultant products was characterized by X-ray diffraction (XRD) using an X'Pert-ProMPD (Holand) D/max-γA X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm) at a scan rate of 5°min −1 .The near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) experiments were conducted on a laboratorybased SPECS near-ambient pressure XPS system.X-ray photoelectron spectroscopy (XPS) measurements were performed using a Nexsa instrument (Thermo Fisher Scientific) with Al Kα radiation (hυ = 1486.6eV) as the excitation source.Scanning electron microscopy (SEM) images were performed by a FEI-quanta 200 scanning electron microscope with an acceleration voltage of 20 kV.The electron paramagnetic resonance (EPR) analysis was performed on a Bruker EPR JES-FA200 spectrometer.Transient fluorescence decay spectra were characterized on an Edinburgh FLS1000 fluorescence spectrometer.The ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) was recorded on a Shimadzu UV-3600 spectrometer.Atomic force microscopy (AFM) was performed by means of a Veeco DI Nanoscope Multi Mode V system.The isotopically labeled experiments were carried out on a gas chromatography-mass spectrometry (GC-MS, Agilent 7890B-5977B).

AQY calculations
The AQYs at different wavelengths (λ = 365-850 nm) for ːPDI 2− /PDI 2− catalysts were determined by taking catalyst solution (0.05 g catalyst and 50 mL deionized water) irradiated by a 300 W Xe lamp applying different band-pass filters for 5 h.The average intensity of irradiation was determined by a CEL-NP2000-10A spectroradiometer (Beijing China Education Au-light Technology Co. Ltd, China) and the irradiation area was 4 cm 2 (2 cm × 2 cm).For example, when λ = 550 nm, the average intensity of irradiation was determined to be 269.8mW cm -2 .The amount of H 2 generated in 5 h was 87.5 μmol.The number of incident photons (N) is as calculated by the following equation: Here, R H 2 : The H 2 evolution rate (mmol −1 ); ΔG r : The reaction Gibbs energy of water splitting (J mol −1 ); P Sun : Light power density of the AM 1.5 G standard solar spectrum (100 mW cm −2 ); S: The irradiation area.

Fig. 1 |
Fig. 1 | Schematic comparison between different mechanisms.a The classic triplet-fusion upconversion pathway in an organic sensitizer-annihilator system through ISC, and b in this work, the two-band excitation upconversion pathway in a spin-hybrid :PDI 2-/PDI 2-all-organic semiconductor to utilize the full vis-to-NIR solar spectrum.

Fig. 3 |
Fig. 3 | Structural characterizations of self-assembled :PDI 2− /PDI 2− organic semiconductor.a SEM images to show morphology evolution of •PDI 2− during the self-assembly.b AFM images of :PDI 2− /PDI 2− samples after 24 h and 120 h selfassembly and corresponding line-scan profile to show the thickness of :PDI 2− /PDI 2− nanobelt.c Single-crystal SAED pattern of :PDI 2− /PDI 2− nanobelts (120 h).d A simulated arrangement model of :PDI 2− /PDI 2− crystal according to the SAED and XRD patterns.e XRD patterns and ESR spectra of :PDI 2− /PDI 2− with different selfassembly times within 120 h.f Profiles of the changing a-axis length and 2θ diffraction angle during the self-assembly were calculated according to the {100} peak position in XRD patterns.